Please allow me to return to the HDCD question, even though HDCD is something like an enigma and a scam... (both quotes from the web).

The HDCD patent was issued in 1997 (to Pacific Microsonics). It has expired in 2017, at least according to all information on the web. Thus it should no longer be protected. I did understand MPD was not to be endangered in any way by including copyright-protected items, but AFAIK this no longer holds for HDCD.

So, please, let me ask you, Max: Please reconsider putting an HDCD decoder into MPD. I did read ffmpeg has such a module, and there is the original hdcd.exe that was reverse-engineered from Windows Media Player. I have read a huge number of web publications on HDCD, and I understand it is something quite awful. But I feel HDCD today still is one of the big "white spots" on the MPD landscape.

Remark: As I said above, there is a good number of HDCD discs out there (e.g. the entire Grateful Dead reissue catalog since many years). Best practice for ripping with dBpoweramp is to rip such discs as-is, without decoding, thus preserving the original AccurateRip checksum for future checks. Yes, on could afterwards convert all such 16/44.1 HDCD rips with dBpoweramp Music Converter to 24/44.1 (it uses hdcd.exe). However, this would store HDCDs twice on one's hard disk, wasting space. In the age of hi-res audio downloading, hard disk space is becoming precious again.

It only needs to be told to direct HDCD files to the FFmpeg decoder plugin.

There is no such thing as an HDCD file, only 16/44.1 PCM files in whatever coding was selected (FLAC, WAV, etc).

On Windows, foobar2000 does decode HDCD within 16/44.1 PCM "on-the-fly" via a plugin (component), and - quite important - it lets you put the technical details (PE, LLRE, TF) into the status line. dBpoweramp can do "off-line" conversion from 16.4/44.1 to 20/44.1 using the hdcd.exe reverse-engineered tool.

Unfortunately, it is over my head, knowledge and time to go into the MPD source code myself and try to figure out what would have to be done to let MPD look at a 16/44.1 PCM stream from a CD/FLAC/whatever and detect the bit pattern that would announce embedded HDCD codings. I am awfully sorry for that.

There's nothing for me to consider, because nobody submitted any code to me.

Please, what would you recommend now? Post a feature request on the github MPD site and find voluntary help from the project community? I see that you yourself do a considerable portion of the posts at github. Maybe I didn't understand your role yet, maintainer for MPD, or an active developer, or both at the same time.

I personally have no interest in this bullshit codec, and I have no interest in adding support for this "feature" to MPD.

If anybody wants a feature, he can either write code and send it to me, or convince me (or somebody else) to write it. The latter won't happen with HDCD, because, as I said, it's bullshit, and I don't like wasting my free time with bullshit features, sorry.
But maybe you can find somebody else to do it and send code to me.

This is how open source works. If you don't like that principle, don't use it. Or don't use MPD if you don't like that one of its developers considers HDCD bullshit. Whatever. It's your freedom.

So far as the player is concerned, there is no difference between HDCD and CD. The dither applied to the LSB of an HDCD has a pattern to it which can be recognised and used to make mysterious adjustments to the HDCD filter in the player, if it has one. Whether these functions were ever actually implemented seems open to doubt, but if a PM converter was used to master the CD it means that the studio cared enough to use the best converter available, so the CD probably sounds better than average in any case. Use of the PM converter activates the HDCD indicator pin of the filter chip, so it is a good way to check that your output is bit perfect but does not mean the CD actually has any HDCD enhancements.

Ripping to a flac file does not alter the LSB pattern so, if you have an HDCD DAC, the light will still come on.